A digital-coherent optical reception scheme receives a light signal in combination of coherent receiving and digital signal processing. In recent years, an optical receiver conforming to digital coherent scheme in optical-fiber communication has advantages of Optical Signal to Noise Ratio (ONSR) tolerance, dispersion tolerance, and Polarization Mode Dispersion (PMD) tolerance and has prospect of reduction of cost for apparatus units.
For example, this optical receiver causes a light signal transmitted through the optical fibers to interfere with local light so that in-phase signal light I and quadrature-phase signal light Q are obtained. The obtained signal lights are converted into electric signals and then data is demodulated through digital signal processing through the use of a Digital Signal Processor (DSP).
Here, improvement in quality of a receiving signal leads to improvement in performance of the optical receiver and further leads to improvement in performance of the optical-fiber communication system. Additionally, in order to ensure the increase in transmission capacity, maintaining the fine signal quality is an important index.